C # Multithreading multiple threads handles one task and then executes the rest

In the comments, I is told that I should avoid thread.sleep (), all right, let's try this:
1. You could wrap the Parallel.ForEach within a task and manually add your Completedcallback ()

 Public Task Doworkasync (doworkcompletedcallback completedcallback) {    return  Task.Factory.StartNew (    {         Parallel.ForEach        //callcallback manually         Completedcallback ();    });}

2. Use ContinueWith

Task.Factory.StartNew (    {         //do work    }    ). ContinueWith (completedcallback);

3. You could assign callback in the caller, it ' s the same as 2.

Task NewTask = Task.Factory.StartNew (    {         //do work}); Newtask.continuewith (oncompleted);

4. Consumer-publisher Patterns
Producer Consumer problem

Assume you has a huge object hosting messages Concurrentlist<string>, task pushes a message ' oncompleted ' to the OB Ject, on the other side, a listener subscribes the message, once a oncompleted received, it'll execute the continuation.

C # Multithreading multiple threads handles one task and then executes the rest